Solid propellant charge making in mold having perforated separator means

ABSTRACT

A method of casting a plurality of solid propellant charges comprising locating in a mould cavity a plurality of quantities of casting powder spaced apart by separator members, compressing the casting powder, displacing air from the casting powder by a casting liquid and curing the propellant charges formed from the quantities of casting powder.

I Umted States Patent 1151 3,678,138 Gordon et al. [4 1 July 18, 1972 54] SOLID PROPELLANT CHARGE 149/97 MAKING IN MOLD HAVING PERFORATED SEPARATOR MEANS [561 [72] Inventors: Stuart Gordon, Cleobury Mortimer; UNITED STAES PATENTS gr hn l y; Alwyn 3,191,s3s 6/1965 Mulloy ..264/3 R llffe. Klddermmsler, all of England 3,359,350 12/1967 Godfrey [73] Assignee: Imperial MM Mm (Kymh) 3,413,384 11/1968 Olliff ..264/3 R Limited Birmingham England Primary Examiner-Carl D. Quarforth [22] Filed: March 30, 1970 Assistant Examiner-Stephen J. Lechert, Jr. [2 1] App] No 2 24 Anomey-Cushman, Darby & Cushman 1 [57] ABSTRACT Foreign l l one A method of casting a plurality of solid propellant charges comprising locating in a mould cavity a plurality of quantities Apnl 3, 1969 Great Bntam ..l7,482/69 of casting powder spaced m y separator members pressing the casting powder, displacing air from the casting [52] U.S.Cl. ..264/3R, 149/14, 149/95, powder by a casfing liquid and curing the propellant charges [51 1 l L CI r06: fonned from the quantities of casting powder.

11 [58] Field of Search ..264/3 R; 149/14, 94, 95, 96, [0 Claim, 3 swing figures Patented July 18, 1972 3,678,138

3 Sheets-Sheet 2 34 F|G.2. I

SOLID PROPELLANT CHARGE MAKING IN MOLD HAVING PERFORATED SEPARATOR MEANS BACKGROUND OF THE INVENTION This invention relates to methods of casting solid propellant charges for rocket motors and to charges produced by such methods.

Solid propellant charges for rocket motors are normally produced by casting methods well-known to those skilled in the art. When the required propellant charge is a simple right cylinder, there are two alternative casting methods of which the first is to cast each charge in a corresponding mould, and of which the second is to cast multiple lengths simultaneously, followed by the severing of the casting into the required lengths. For both of these alternative methods, it may be necessary to trim the end faces of each charge in order to remove any imperfectly cast portions thereof.

However, many desirable configurations for cast propellant charges are not simply right cylinders with planar ends, but are provided with a right cylindrical portion with an extension of a non-planar end form. Examples of typical non-planar end forms are truncated cones and castellations. The particular end form selected is dictated by the ballistic properties required from the charge.

In the manufacture of this form of charge configuration, there are again two alternative methods. In the first of these, there are cast simple right cylinders either in unit or multiple lengths, and each of these is subsequently machined to the required end form. This method is time consuming in tenns of the number of operations required in any significant number of units which are cast singly, and is, in any case, wasteful of materials and necessitates the hazards of machining operations on solid propellant charges.

The second alternative is the casting of the solid propellant charge in its desired final configuration as described and claimed in our co-pending British Pat. application No. 25977/66 (Ser. No. 639176 With this method no machining is required and there should be no imperfectly cast portions of the charge.

Although the methods described and claimed in our copending patent application referred to above are basically satisfactory, they are directed to the casting of the solid propellant charges one at a time, so that it is an object of the present invention to provide a method of casting solid propellant charges in which a plurality of charges is cast in a single casting operation.

SUMMARY OF THE INVENTION In accordance with the invention, a method of casting a plurality of solid propellant charges for rocket motors comprises locating in a mould cavity a plurality of quantities of casting powder of which each isspaced from each neighboring quantity by a corresponding separator member extending across the mould cavity and having apertures through the separator member; compressing the quantities of casting powder with any corresponding displacement of the separator member or members by ram pressure from at least one end of the mould cavity; displacing air from interstices between the granules of casting powder by the introduction of a casting liquid at one end of the mould cavity, and the passage of the liquid through the powder via the apertures of the separator member or members; curing the propellant charge formed from each quantity of casting powder to a solid mass, the ram pressure being maintained at a substantially constant value from the first application of ram pressure until the curing is completed; and removing the plurality of solid propellant charges from the mould cavity.

Preferably the mould cavity is formed in a mould which is capable of being split longitudinally thereof to facilitate the removal of the solid propellant charges from the mould cavity.

Preferably also the mould cavity is provided with a lining of a combustion inhibitor material prior to the location therein of the plurality of quantities of casting powder.

Preferably also the or each separator member is either provided with a parting layer on its surfaces facing the quantities of casting powder to prevent the bonding of the solid propellant charges to said separator member, or alternatively the or each separator member is constructed fi'om a plastics material which will not bond to the solid propellant charges.

Preferably further the casting liquid is introduced to the mould cavity under pressure.

Preferably further the pressure is applied to the casting liquid by a gas inert to the casting liquid and the casting powder and under a pressure of 5 p.s.i. (equivalent to 0.35 7 kg/cm).

Preferably further the casting liquid is desensitized nitroglycerine and the inert gas is nitrogen.'

Preferably further the casting powder is principally nitrocellulose and the propellant charge formed fi'om each quantity of casting powder is cured at 100- F (38- 71C- for 144 24 hours).

Preferably further the ram pressure is sufficient to provide a pressure on the casting powder to compress the granules thereof at between lOand 1,000 p.s.i. (equivalent to 0.7 70 kg/cm).

The invention also consists in a solid propellant charge for a rocket motor manufactured in accordance with any of the methods of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS A typical embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a partly sectioned elevation of a first loaded mould assembly for casting a plurality of solid propellant charges;

FIG. 2 is an elevation similar to FIG. I of a second loaded mould assembly; and

FIG. 2A is an enlargement of the portion X on FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, a mould assembly 1 comprises two halves 2,3 which part along a plane extending longitudinally of the mould assembly 1. The halves 2,3 are secured together by bolts 4 passing through bolt holes 5 provided in two flanges 6, each flange extending from a corresponding one of the halves 2,3.

The mould assembly 1 includes a base plate 7 to which the halves 2,3 are bolted as illustrated at 8. The base plate 7 is provided with an annular drainage channel 9 leading through passages 10,11 to a casting liquid outlet nipple 12.

The mould assembly 1 also includes a head assembly 13 which comprises a pneumatic ram 14 which consists of a piston 15 (shown in its lowest position) from which extends a projection 16 of which the upper end is provided with an air supply nipple 17, and also a piston rod 18. The air supply nipple 17 is connected to an air cylinder 19 within which is reciprocable the piston 15, via passages 20. The air cylinder 19 is also provided with an air bleed 21 opening into the cylinder 19 on the side thereof spaced from the passages 20 by the piston 15.

The piston rod 18 is connected through a union 22 to a head plate 23 which forms the lower limit of casting liquid chamber 24. The head plate 23 is provided with a number (e.g. 32) of apertures 37, each typically having a diameter of three sixtyfourths of an inch l.l9 mm). The casting liquid supply chamber 24 is defined by the head plate 23, by the adjacent internal walls of the halves 2,3 and by a plate 25 through which passes the piston rod 18. The piston rod 18 is provided with suitable sealing rings 26 and sealing rings 27,28 are provided between the projection 16 and a top plate 29, and the top plate 29 and the wall of the air cylinder 19, respectively.

The casting liquid supply chamber 24 is provided with a casting liquid inlet 30 connected through passages 31 to a casting liquid supply nipple 32.

The casting liquid chamber 24 is also provided with an inlet 34 projecting from the plate and connected to the exterior of the head assembly 13 by a passage 35. The passage 35 is provided with a gas inlet nipple 36, whereby inert gas can be supplied under pressure to the chamber 24.

The head assembly 13 is bolted to the upper ends of the halves 2,3 as illustrated at 33.

The halves 2,3 define a mould cavity 40 (shown in FIG. 1 containing propellant charges to be described later) of which the ends are determined by the head plate 23 and the base plate 7. Between those ends the mould cavity 40 is a simple right cylinder. Provided within the mould cavity 40 are a plurality of separator members 41 which have a function to be described later and of which each consists of a shaped upper surface 42 and a flat lower surface 43 connected together by a number (e.g. 32) of apertures 44 of which each has a typical diameter of three sixty-fourths inch (1.19 mm). Each separator member 41 is provided immediately adjacent the surface 43 with a separator membrane 45 which is in the form of a simple disc in this example but can have its lower surface shaped as required. Each membrane 45 is provided with apertures 46 which correspond to, and are located in registration with, the apertures 44 through the separator member 41. Each separator member 41 and its associated separator membrane 45 are conveniently formed from compatible plastics materials such as polyethylene terephthalate, polypropylene, polyacetate resin, polycarbonate resin, polyamide resins or polytetrafluoroethylene, as examples. The material to be chosen must be one which will not bond to the propellant with which it is to be used. Alternatively each separator member 41 and separator membrane 45 may be manufactured from suitable metals, provided that they are given surface coatings of suitable parting agents. The edges of the meeting faces of the separator members 41 and membranes 45 are chamfered to provide an annular notch 49.

The mould cavity 40 also (although not necessarily) containsa tubular lining (not shown) of a suitable combustion inhibiting material, for example, cellulose acetate which is transparent or a combustion inhibiting elastomeric compound.

In use the mould assembly 1 is assembled together by the bolting together of the halves 2,3 and the mounting thereof on the base plate 7. The mould cavity 40 is provided with the tubular lining of combustion inhibiting material if required, and there is located at the lower end of the mould cavity 40 one of the separator members 41 with its corresponding separator membrane 45. At this stage the head assembly 13 is not attached to the upper end of the mould assembly.

There is then loaded into the mould cavity 40 the first quantity of casting powder, which in this example consists of chopped, extruded, strands characteristically of nitrocellulose, each of the resulting granules having a diameter and length of about 0.03-0.05 inch (0.76-1.27 mm). This first quantity is then levelled and lightly compressed by a hand operated tool.

The next separator member 41 and separator membrane 45 are then passed into the mould cavity 40 from the upper end thereof, and are slid towards the lower end to rest on top of the first quantity of casting powder.

A second quantity of casting powder, which will normally, but not necessarily, be the same powder as that used for the first quantity, is loaded into the mould cavity 40 and is again levelled and compressed on top of the second separator member 41.

This process is repeated to approximately fill the mould cavity 40 with a series of quantities of casting powder of which each is spaced from each neighboring quantity by a corresponding separator member and separator membrane.

It is to be noted that it is not essential that the same casting powder be used in each quantity, provided that casting can be accomplished with the use of the same casting liquid for all of the quantities, and the use of the same pressures, temperatures and curing times. In addition it is unnecessary for the various quantities to be of the same size. Also any or every quantity of casting powder may comprise a series of aliquots of differing compositions so as to provide a series of differing thrust levels from the corresponding final product charge.

When the loading of the mould cavity 40 has been completed, it is arranged that the level of the uppermost quantity of casting powder is very close to the upper end of the mould cavity 40, and with the piston 15 in its uppermost position, whereby the head plate 23 attached thereto is at its highest position, the head assembly 13 is bolted onto the upper ends of the halves 2,3 by the bolts 33.

The casting powder is then compressed by the supply of compressed air through the air supply nipple 17 into the air cylinder 19 to drive the piston 15, and hence the head plate 23, downwardly with a pressure applied to the casting granules within the range 10 1,000 p.s.i. (0.7 70 kg/cm) as desired, typically 50-150 p.s.i. (3.5 10.5 kg/cm). This will operate upon the uppermost quantity of casting powder, and through that powder and the uppermost separator member 41 and separator membrane 45, to the next uppermost quantity of casting powder and so on throughout the entire series of quantitles of casting powder. This will entail some degree of displacement of all of the separator members 41 with their membranes 45, but their transverse dimensions are arranged to be such that they are readily slidable within the tubular inhibitor lining. The sizes of the granules of casting powder are such that they will not pass readily through the apertures 44,46, and they are rendered immobile by the applied pressure.

The casting liquid chamber 24 is then supplied with desensitized nitroglycerine through the supply nipple 32, the passage 31 and the inlet 30 to such a degree that there results an ample pool of desensitized nitroglycerine on top of the head plate 23. This is achieved by the supply of nitrogen to the upper surface of the desensitized nitroglycerine reservoir connected to the nipple 32, as is well-known to those skilled in the art. The nitrogen is at a pressure above that of the atmosphere which lies within the range 5 p.s.i. (0.35 7 kg/cm), preferably 15-50 p.s.i. (l.05-3.5 kg/cm). This pressure will operate to force nitroglycerine through the apertures 37 into the uppermost quantity of casting powder. Because of the compressed nature of the granules of casting powder, the nitroglycerine will progress through this quantity with an almost perfectly horizontal front, and will drive all air out of the interstices between the granules. While this is being done the supply of nitroglycerine to the chamber 24 ensures that the upper surface of the head plate 23 is always covered with nitroglycerine. This prevents the passage of any gas in the chamber 24 into the casting powder with the nitroglycerine.

When the front of desensitized nitroglycerine reaches the uppermost separator member 41, having driven all of the interstitial gas out of the uppermost quantity of casting powder, the liquid will pass through the apertures 44,46 into the next uppermost quantity of casting powder and will again form an almost perfect horizontal front permeating through this quantity of casting powder.

The nitroglycerine will progress in this way through each of the quantities of casting powder until it reaches the lowest separator member 41 and membrane 45 whereupon nitroglycerine will progress into the annular channel 9 and from this through the passages 10 and 1 1 to be emitted at the outlet nipple 12. When this occurs the operator will observe that only a small quantity of excess nitroglycerine will be exhausted through the nipple 12 before there is no longer any indication of gas bubbles which means that all interstitial gas has been displaced by the casting liquid. The outlet nipple 12 is then sealed by any convenient method, and the supply of nitroglycerine to the chamber 24 is terminated, although there is maintained a continuous pool of nitroglycerine on top of the head plate 23. A nitrogen supply is commenced through the nipple 36 at the same head as that supplied to the nitroglycerine reservoir, and the pressure exerted by the head plate 23 is maintained by the continuation of the air pressure in the cylinder 19 at the required level.

Using desensitized nitroglycerine and nitrocellulose, 30 to 40 minutes can elapse from contact therebetween before the passage of nitroglycerine is obstructed by the swelling of the nitrocellulose granules, such that the length of the mould cavity 40 and the pressure applied to the nitroglycerine are governed by the need to complete the supply of nitroglycerine within this time. In practice mould cavities of 3 to 6 meters in length can readily be accommodated and practical handling considerations become the main limitation.

Contraction of the nitrocellulose casting powder mass occurs during absorption of the desensitized nitroglycerine by the powder, the pressure exerted by the head plate 23 being maintained by continuous addition of air through the air supply nipple 17 corresponding to the progression of the shrinkage. Swelling of the powder eventually results on account of the physical interaction between the powder and the liquid so that the head plate 23 is forced to retract and air is forced through the nipple 17 with the selected pressure maintained. During the swelling (gelling) of the powder, the granules tend to coalesce. Simultaneously a bond is formed between the gelled propellant mass and any inhibitor material. Curing is effected by heating the propellant mass to a temperature between l00-l60 F (38-7l C), typically 150 F(65 C) for a period between 144-24 hours, typically 96 hours causing thermal expansion and subsequently compression and consolidation followed by cooling and contraction, which is also accommodated by movement of the pressure plate. Air and nitrogen pressures are maintained at the required levels throughout curing.

After curing the head assembly 13 is removed and the mould halves 2,3 are demounted from the base plate 7 and are separated from each other. This releases the resulting cast solid propellant charges which are still located as a series with the separator members 41 and separator membranes 45 between each cast charge, all of this lying within the common tubular lining of inhibitor material. The tubular inhibitor lining is then covered at the indentation arising at each notch 49 between each charge and the charges are separated from the intervening separator members and separator membranes 45. The combustion inhibiting material can be trimmed more accurately after each solid propellant charge has been produced and any inhibition of the ends of the charges which is required can subsequently be effected.

In modifications of this example, it is not necessary for the mould cavity 40 to contain the combustion inhibiting material, but rather that the solid propellant charges be cast without any inhibitor material and if required this be applied subsequently by methods known per se.

In addition it may be possible to replace the mould halves 2,3 by a complete tubular member in which case the cast assembly of charges and separator members and separator membranes 41,45 can be expelled from one end of the disassembled mould.

Further it may be preferred to apply compressive pressure to the quantities of casting powder from both ends of the mould cavity 40 rather than from the upper end as has been described above.

Also the supply of casting liquid can be reversed in direction, liquid being supplied through the nipple l2 and exhausted through the nipple 32.

Referring now to FIGS. 2 and 2A of the drawings, these show a second example of the invention which is similar to the first, such that the same reference numerals have been applied to like parts, but is modified because the mould cavity 40 contains a core pin 60.

The core pin 60 is provided at its lower end with a spigot 61 which is screw-threaded at 62 and clamped by a nut 63 to a bottom plate 64. The bottom plate 64 is located at the bottom of the mould cavity 40 and is provided with typically 32 holes of which the ends open into two annular channels 65 in each face of the bottom plate 64. The channels 65 in the lower face of the bottom plate 64 connect with the'casting liquid drain passages 10,11.

From the bottom plate 64 the core pin 60 is cylindrical and extends coaxially with the mould cavity 40 to the upper end 66 of the core pin 60. The end 66 is slidably received within a pressure member 67 bearing upon the head plate 23 which, in this example, is annular and surrounds the core pin 60. The pressure member 67 is borne upon by the piston rod 18 through the union 22.

Because of the provision of the core pin 60, each separator member 41 is also annular, and in this example is provided with upper and lower separator membranes 45 The apertures 44 through the separator members 41 are also provided with annular channels 68 into which open the apertures in the membranes 45.

The notches 49 are replaced by peripheral grooves 69 in the separator members 41. The separator members 41 are conveniently of polypropylene with relatively flexible polythene for the membranes 45.

This example is used for casting a plurality of solid propellant charges in the same way as that described in relation to FIG. 1, except that the core pin 60 and attached bottom plate 64 are provided in the mould cavity 40 before the location therein of the first separator member'4l and its membranes 45. In addition, of course, tubular charges are produced.

The core pin need not have a circular cross section but can be of any ballistically advantageous cross-section, for example oval or star shaped with any required number of star points.

We claim:

1. A method of casting a plurality of solid propellant charges for rocket motors comprising locating in a mould cavity a plurality of quantities of casting powder of which each is spaced from each neighboring quantity by a corresponding separator member extending across the width of the mould cavity and having apertures through the separator member; compressing the quantities of casting powder with any corresponding displacement of the separator member or members by ram pressure from at least one end of the mould cavity; displacing air from interstices between the granules of casting powder by the introduction of a casting liquid at one end of the mould cavity and the passage of the liquid through the powder via the apertures of the separator member or members; curing the propellant charge formed from each quantity of casting powder to a solid mass, the ram pressure being maintained at a substantially constant value from the first application of ram pressure until the curing is completed; and removing the plurality of solid propellant charges from the mould cavity.

2. A method according to claim I including the step of splitting the mould longitudinally after the curing step to facilitate the removal of the solid propellant charges from the mould cavity.

3. A method according to claim 1 including the step of providing the mould cavity with a lining of a combustion inhibitor material prior to the location therein of the plurality of quantifies of casting powder.

4. A method according to claim 1 including the step of providing the or each member with a parting layer on its surfaces facing the quantities of casting powder to prevent the bonding of the solid propellant charges to said separator member.

5. A method according to claim 1 wherein the or each separator member is constructed from a plastics material which will not bond to the solid propellant charges.

6. A method according to claim 1 wherein the casting liquid is introduced to the mould cavity under pressure.

7. A method according to claim 6 wherein the pressure is applied to the casting liquid by a gas inert to the casting liquid and the casting powder and under a pressure of 5 p.s.i. (equivalent to 0.35 7 kg/cm*).

8. A method according to claim 7 wherein the casting liquid is desensitized nitroglycerine and the inert gas is nitrogen.

9. A method according to claim 8 wherein the casting powder is principally nitrocellulose and the propellant charge formed from each quantity of casting powder is cured at 100 F (38 71C) for 144 24 hours.

10. A method according to claim 1 wherein the ram pressure is sufficient to provide a pressure on the casting powder to compress the granules thereof at between 10 and 1,000 p.s.i. (equivalent to 0.7 70 kg/cm).

i ll l 5 

2. A method according to claim 1 including the step of splitting the mould longitudinally after the curing step to facilitate the removal of the solid propellant charges from the mould cavity.
 3. A method according to claim 1 including the step of providing the mould cavity with a lining of a combustion inhibitor material prior to the location therein of the plurality of quantities of casting powder.
 4. A method according to claim 1 including the step of providing the or each member with a parting layer on its surfaces facing the quantities of casting powder to prevent the bonding of the solid propellant charges to said separator member.
 5. A method according to claim 1 wherein the or each separator member is constructed from a plastics material which will not bond to the solid propellant charges.
 6. A method according to claim 1 wherein the casting liquid is introduced to the mould cavity under pressure.
 7. A method according to claim 6 wherein the pressure is applied to the casting liquid by a gas inert to the casting liquid and the casting powder and under a pressure of 5 - 100 p.s.i. (equivalent to 0.35 - 7 kg/cm2).
 8. A method according to claim 7 wherein the casting liquid is desensitized nitroglycerine and the inert gas is nitrogen.
 9. A method according to claim 8 wherein the casting powder is principally nitrocellulose and the propellant charge formed from each quantity of casting powder is cured at 100* - 160* F (38* -71*C) for 144 - 24 hours.
 10. A method according to claim 1 wherein the ram pressure is sufficient to provide a pressure on the casting powder to compress the granules thereof at between 10 and 1,000 p.s.i. (equivalent to 0.7 - 70 kg/cm2). 